Metal Matrix Composite materials are designed to surpass monolithic metals in the areas of modulus, temperature resistance, strength, hardness, conductivity, dimensional dampening, and weight. MMC composites can take many different forms with variations in the type, amount, and nature of the fibers used for reinforcing and the choice of the matrix material. The most common forms of MMC found in the market place today are aluminum based composites with various amounts of silicon carbide or tungsten carbide particles. A die cast material with 15% or 20% silicon carbide is being marketed as a replacement for cast iron in a number of aircraft and automotive applications. Also, a MMC based on 50% to 70% silicon carbide particles is being used for light weight enclosures.

Drilling MMC

The principle problem in drilling MMC is tool life. The silicon carbide fibers quickly dull the cutting edge of tungsten carbide tooling. PCD tooling lasts much longer, provides closer hole sizes and a good finish.

In one recent study, A Precorp PCD drill was able to cut 300 inches of a 20% silicon carbide filled aluminum. A carbide drill was dull after drilling less than 2 inches of the same material.

Studies using Precorp's Series 70 DiaEdge drills have shown that the cost of drilling MMC's with PCD drills is from 1/3 to 1/4 of that of using carbide drills prior to including cost savings related to increases in productivity and improvements in hole quality.

Speeds and Feeds for MMC

The best tool life with PCD in MMC's is achieved with aggressive feed rates that allow the tool to complete the hole with the minimum linear travel in the material. The same study showed that modest feed rates cause an insert PCD drill to fail.

Practical feed rates per revolution, however, are limited by the strength of the MMC material, the drilling speed, the depth of the hole, and the diameter of the drill. Experience has shown that a good starting point for a 1/4in drill in MMC's with flood coolant is at 200 SFPM with .004 in per revolution. Feed rates per revolution can be increased with a lower particle content, with high cutting speeds, or increases in drill diameter.

Feed rates should be reduced as hole depth increases or drill diameter decreases.In one recent study with Duralcan's F3S.20S-T6, (an aluminum based MMC filled with 20% SiC), speeds of 492 SFPM combined with feeds of 75 IPM provide the lowest wear rate for a .2510 diameter PCD Drill.

Duralcan states in their machining guidelines: 'By far the most cost effective cutting tool material is diamond. Carbide tools should not be used except in operations for which diamond tools do not exist, or for non production tasks such as roughing out prototypes.'

Duralcan also recommends PCD-veined drills by Precorp. 'Their unique construction offers the ultimate in abrasion resistance, dimensional accuracy, and surface finish. Reaming is usually not required after using these drills.'

The above information was taken from:
'Drilling Advanced Compositions with Diamond Veined Drills'
by John Bunting, President of Precorp Inc.

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